(i) Field of the Invention
The present invention relates to a polishing method and a polishing device, particularly to a polishing method and a polishing device which can preferably be used for polishing an inner peripheral end surface or another surface of a glass substrate for a magnetic recording medium, and the like.
(ii) Description of the Related Art
An aluminum substrate has been broadly used as a substrate for a magnetic disc or another magnetic recording medium, but accompanying miniaturization/thinning of the magnetic disc and densification of recording, the aluminum substrate is increasingly replaced with a glass substrate which is superior to the aluminum substrate in surface flatness and substrate strength.
In general, a glass substrate chemically reinforced to raise the substrate strength, or a crystallized glass substrate whose strength is raised by crystallization is used as the glass substrate for the magnetic recording medium.
Moreover, for a magnetic head, high-density recording brings about a transition from a thin-film head to a magnetic resisting head (a magnetro-resistive head) (MR head) and a giant magnetic resisting head (a giant magnetro-resistive head) (GMR head). Therefore, it is expected that regeneration of the magnetic recording medium using the glass substrate with the magnetic resisting head becomes a mainstream from now on.
In this manner, various improvements are added to the magnetic disc for high-density recording, and with the progress of the magnetic disc, new problems in the glass substrate for the magnetic recording medium arise one after the other. One of them is high-level cleaning of a glass substrate surface. Specifically, foreign particles sticking to the glass substrate surface may cause defects of a thin film formed on the glass substrate surface, or form convex portions on a thin film surface. An adequate glide height cannot be obtained.
Moreover, in the regeneration of the magnetic recording medium using the glass substrate with the magnetic resisting head, if the flying height of the head is lowered to enhance recording density, regeneration error may occur, or regeneration cannot be performed. Such disadvantageous phenomenon is caused by thermal asperity resulting from the convex portions formed on the magnetic disc surface by the particles on the glass substrate. In this case, heat is generated on the magnetic resisting head (the magnetro-resistive head), the resistance of the head is fluctuated, and electromagnetic conversion is adversely affected.
The cause of the foreign particles on the surface of the glass substrate for the magnetic recording medium lies in that a glass substrate end surface having no smooth state grinds against a wall face of a resin case. Resin or glass particles generated by the grinding, and other particles caught by inner and outer peripheral end surfaces of the glass substrate stick to the surface. Inventors et al. have found that especially the inner peripheral end surface of the glass substrate is coarser than the outer peripheral end surface, more easily catches particles, and obstructs the high-level cleaning of the glass substrate surface.
Additionally, proposed is a technique of removing cracks generated on an end surface portion of a glass substrate by chemical etching to enhance a substrate strength (Japanese Patent Application Laid-open No. 230621/1995). In this case, the depth of the crack is decreased, but the crack is etched, enlarged and indented to easily catch particles, which further obstructs the high-level cleaning of the glass substrate surface. Moreover, in the chemical etching, it is difficult to control the surface precision of the end surface portion at a high level. Furthermore, it is difficult to completely remove cracks, and only an insufficient deflection strength is provided.